A quick glance online shows multiple reports of Diablo 3 accounts being hacked, suggesting it is a growing issue. Players have seen the loss of items stored in character inventory and stash, and gold removed as their accounts are stripped bare. Blizzard has been working to "roll back" affected characters to a point before accounts were compromised, but some progress is lost.

A solid cause is not found but most say it is a session id hack. They get your session id via joining a public game and then fake their client to say it is your session id and can access your last played toon.